Acoustical laminate

ABSTRACT

An acoustical laminate comprising multi-ply glass fabric impregnated with an epoxy resin and a method of manufacturing the laminate. The laminate may be bonded to suitable backing, such as a honeycomb core with a solid backing, to produce an acoustical panel. The panel provides excellent, substantially linear, sound absorption characteristics.

BACKGROUND OF THE INVENTION

This invention relates to acoustic laminates and panels and morespecifically, to laminates comprising epoxy resin impregnated glassfabrics.

A variety of acoustic sandwich constructions are known in the art. Thesegenerally consist of an impervious backing sheet and a porous face sheetseparated by a foam or compartmented layer, such as a honeycomb core.Typical of these acoustic panels is that disclosed by B. T. Hulse etal., in U.S. Pat. No. 3,166,149 and T. S. Crispin et al, in U.S. Pat.No. 3,822,762. Often these panels include a porous cover sheet which,while sometimes decorative, adds undesirable weight to the assemblywhile reducing the acoustic efficiency of the system.

While some of the prior art acoustic laminates and panels have beenrelatively effective, problems remain both in the structures and methodsof manufacture. Resin characteristics, such as quantity of resin, flowand wetting characteristics, etc., affect panel properties. In somecases resin may block pores, reducing acoustic properties, while inothers which limit resin flow to a point where porosity is retained,structural characteristics of the acoustic laminate are poor, resultingin delamination or unbonded conditions when the panel is subjected toloading.

Polyimide resin impregnated glass fabric acoustic laminates of the sortdescribed in U.S. Pat. No. 3,502,171, have been used, especially in hightemperature applications. However, these materials have a number ofdisadvantages when their high temperature characteristics are notrequired. Because of the high temperature cures required and the releaseof volatiles during cure, these materials cannot conveniently be curedon plaster or other similar tooling. Both the raw material cost and thecost of fabrication (due to the required high temperature cure andpostcure, and the necessarily elaborate bleeder system) for polyimidematerials are undesirably high.

Attempts have been made to use polyester and epoxy resins, which do notrelease significant volatiles upon curing and have inherently greaterflow than polyimides. However, problems have been encountered incontrolling acoustic and/or structural properties as a consequence ofthe basic resin characteristics. Thus, these resins have not come intopractical use.

Therefore, there is a continuing need for improvements in acousticlaminates and panels.

OBJECTS OF THE INVENTION

It is, therefore, an object of this invention to provide an acousticlaminate and method of manufacture thereof overcoming the above-notedproblems.

Another object of this invention is to provide an acoustic laminate andpanel having superior acoustic properties.

A further object of this invention is to provide an acoustic laminateand panel combining high strength with light weight.

A still further object of this invention is to provide an easilyreproducible method for producing high strength, lightweight,acoustically effective acoustic laminates and panels.

SUMMARY OF THE INVENTION

The above objects, and others, are accomplished in accordance with thisinvention by a laminate made by impregnating a plurality of glass fabricsheets with a suitable epoxy resin, curing the resin to the "B-stage",laying up the plurality of sheets to form a stack having selected sheetorientation, and curing the resin to the "C-stage" under suitabletemperatures and pressure conditions. An acoustic panel may then be madeby bonding this laminate to a suitable core and backing assembly.Typically, a honeycomb core with an impervious backing is coated alongthe core outer edges with a suitable adhesive, the laminate is pressedthereagainst and the adhesive is cured.

DETAILED DESCRIPTION OF THE INVENTION

The acoustic laminate and panel of this invention may have any suitableconfiguration and physical characteristics. They may, for example, beflat or curved in one or more planes. In one embodiment, when used inaircraft engine cowling, the laminate and panel may be shaped tocorrespond to an inlet cowl or nose dome shape. The primary factors inselecting fabric type, thickness, number of plies, etc., and epoxy resintype and quantity are airflow resistance, mechanical strength and soundabsorption properties, such as resonant frequency.

The resonant frequency of a resonator may be expressed by the equation:

    Vo = c/2π √ns/Vl.sub.e = c/2π √σ/b l.sub.e

for a honeycomb sandwich where "Vo" is the resonant frequency, le isequal to 1 + 0.8s^(1/2),"s" is the cross-sectional area of the hole, "l"is the length of the hole (i.e., sheet thickness for perforated metal),"n" is the number of holes, "V" is the volume of the cell, "c" is thevelocity of sound, "b" is the core thickness and "σ" is the fractionalopen area. This establishes a relationship between the length of thehole, which is affected by the thickness of the perforated facing, andthe frequency of the sound to be absorbed. Consequently, the particularapplication may affect the desired facing thickness, depending uponwhether the other variables are changed or held constant. Similarly, thepercent open area or porosity of the facing, or the core thickness for ahoneycomb sandwich may be selected to achieve the desired soundabsorption characteristics.

The laminate may be prepared in a manner providing any suitable airflowresistance (measured in rayls) depending upon the selected acousticapplication. A "rayl" is the ratio of the pressure drop in dynes/cm²across a porous medium, to the gas flow velocity in cm/sec. across themedium. Thus, a rayl rating is indicative of porosity or percent openarea of the medium. For most aircraft engine cowling applications,excellent results are generally obtained with an airflow resistancevalue in the range of about 2 to 60 rayls at an airflow velocity ofabout 17 cm/sec. with optimum results dependent upon the characteristicsof the particular engine. The airflow resistance of these laminateswill, of course, be proportionally lower and higher at lower and higherair flow rates, respectively.

The laminate may have any suitable thickness. Best results are generallyobtained where the laminate has a thickness in the range of 0.02 to 0.10inch. Thicker laminates tend to have greater mechanical strength, butbecome undesirably heavy, more difficult and complex to fabricate, andexpensive, while much thinner laminates have insufficient strength.Optimum results for aircraft engine cowling applications have beenobtained with laminates in the about 0.025 to 0.080 inch range.

The laminate includes one or more plies of a suitable glass fabric.While a single ply has advantages in simplicity of lay up and patternuniformity, multiple plies are generally preferred because of ease informing high quality splices between adjoining fabric sheets in largepanels and because a greater range of desired thickness and strength canmore easily be obtained. Open, plain weave glass fabrics are preferredso that a sufficient quantity of resin may be used to provide highstrength without unduly increasing air flow resistance. A leno weave inwhich a thread locks the strands in position is preferred forhomogeneity of porosity across the surface. Where a low rayls number isdesired, a single ply of open weave material could be selected, whilefor higher rayls numbers, multi-ply tighter weave material orthree-dimensional woven multi-layer fabrics would be selected. Multipleplies are preferable given a pseudo-isotropic orientation, e.g., for 3plies; preferred orientation would be 0°, 30°, 60°.

The selected glass fabric may be impregnated with any suitable epoxyresin. Typical epoxy resin include Bisphenol-A resins, novolac epoxyresins, cycloaliphatic epoxies, polyolefin epoxies, etc. Any suitablecuring agent, such as suitable aromatic amines, amides or acidanhydrides, with or without a catalyst may be used. The curing agent isordinarily used in a stoichiometric equivalent ratio. The resins shouldbe capable of being B-staged to a controlled flow range and/or containflow control agents such as pyrogenic or fumed colloidal silica,colloidal asbestos or an elastomer such as acrylonitrile-butadiene in anamount up to about 15 weight percent, based on resin weight.

The quantity of resin, curing agent and any additive used is dependentupon the glass fabric with which it is employed, since some types ofstrands pick up more resin than others. Also, the type of weave affectsthe percentage resin content. Further, the resin flow characteristicsdetermine the limiting range of resin quantity which may be used toproduce a selected airflow resistance. In practice, when a specificcombination of glass fabrics and resin is selected, test samples withdiffering resin quantities will be prepared and tested for airflowresistance to emperically determine the optimum resin quantity andcharacteristics (such as the desirability of additives), and the optimumdegree of advancement or B-staging.

When excessive resin is used, porosity and control of the acousticcharacteristics of the laminate are reduced, while insufficient resinresults in a weak structure subject to delamination in use. In general,depending upon the type fabric, from about 13 to 32 weight percent resinin the dry impregnated laminate, based on glass fabric weight, gives thepreferred combination of acoustic property control and structuralstrength, with optimum results occurring in the 17 to 32 percent range.

The laminate may be formed and used in any suitable manner. Preferably,individual plies of the selected fabrics are impregnated with theselected resin, which is then cured to the B-stage to form a "pre-preg."With some resins, flash-drying for 5 to 15 minutes at a temperature ofabout 200° to 300° F. gives optimum curing to the B-stage.

The individual plies are then laid up on a mold surface in a selectedorientation and the resin is fully cured. Any suitable mold surface maybe used. Typical molds include plaster or aluminum surfaces, generallycoated with a release agent such as a paste wax or fluorocarbon film.During cure the lay-up is preferably covered with a suitable bleedersheet and subjected to vacuum or positive pressure during heating tocure the resin. In some cases, each ply of fabric may be impregnatedwith an epoxy resin which has had its flow characteristics adjusted withadditives, the plies may be laid up on the mold surface and the resindirectly fully cured. With most resins, optimum full cure is obtained byheating to from 200° to 300° F. for about 60 to 120 minutes at apressure up to about 50 psi.

The acoustic laminate of this invention may be bonded to any suitablecore and backing combination to produce an acoustic panel. Preferably,the core is a honeycomb such as metal or reinforced plastic cores havinga thickness of from approximately 0.20 to 2.0 inches, available fromHexcel or American Cyanamid Co., with an impervious backing. The facinglaminate may be bonded, such as by an adhesive, to the core in anysuitable manner. The bonding should be uniform across the core toprovide maximum strength, although the adhesive must be kept off of thelaminate in core open areas, since otherwise airflow and acousticproperties will be adversely affected. Preferably, the outer edges ofthe core are coated with adhesive, the laminate is pressed thereagainstand the adhesive is cured. The impervious backing may be bonded to thecore prior to, contemporaneous with, or after bonding of the acousticlaminate to the core.

The basic steps in a preferred embodiment of the process of thisinvention include impregnating an open leno weave glass fabric with asuitable epoxy resin, curing the resin to the B-stage, laying up two ormore sheets of the fabric on a suitable mold, placing a bleeder packover the fabric, curing the resin to the C-stage, and bonding theresulting laminate to a honeycomb core having an impervious backing toproduce an acoustical structure. The final product, consisting of anacoustic laminate spaced from an impervious sheet by an open (such as ahoneycomb) core, has excellent structural and acoustic properties.

DESCRIPTION OF PREFERRED EMBODIMENTS

Further details of this invention are described in the followingExamples which constitute preferred embodiments of the process andproducts of this invention. Parts and percentages are by weight, unlessotherwise indicated.

EXAMPLE I

Equal amounts of two Bisphenol-A type epoxy resins, Shell Chemical'sEpon 828 and Epon 1001, are blended with an eutectic composition of twoaromatic diamines, p,p'-methylene diamine and m-phenylene diamine,available from Shell Chemical Company under the Curing Agent Zdesignation, in a methyl ethyl ketone solvent. Three sheets of Type 7533glass fabric from Clark Schwebel, Inc., (ECG 75, 1/2 plain weave, 18 ×18 construction) are impregnated with this mixture, then flash dried byheating for about 15 minutes at about 200° F. Resin pickup, dry, isabout 32 percent. The sheets are laid up with the warp of eachsuccessive sheet oriented at 0°, 30° and 60° to one another against atool surface coated with a polyvinyl alcohol and wax treated surface.This lay-up is covered with a perforated fluorocarbon release film,available from E. I. duPont de Nemours under the Teflon trademark, andthree plies of a 181 glass fabric bleeder, then the resin is cured underabout 10 inches of mercury vacuum for about 2 hours at about 200° F. Thecured laminate when tested for airflow resistance at an airflow velocityof about 16.95 cm/sec. gives a value of about 30 rayls (cgs). A core andbacking member is prepared having the same configuration as the acousticlaminate. The core consists of 2.1 pcf aluminum flexcore honeycomb fromHexcel bonded to an impervious backing of F 155-5-F69/1581 epoxyprepreg. The edges of the open honeycomb cells are coated with areticulating epoxy adhesive from American Cyanamid Co. The laminate ispressed thereagainst by a vacuum bag at a pressure of about 24 inches ofmercury and temperature of about 250° F. for about 90 minutes to curethe adhesive and prepreg. A strong, well bonded acoustic panel results,having excellent acoustic properties.

EXAMPLE II

An epoxy resin, a modified Bisphenol-A/novolac epoxy resin blend mixedwith a dicyandiamide curing agent is available under the 5134designation from E. I. duPont de Nemours & Co. to provide an about 250°F curing system. Three sheets of Type 7533 glass fabric are impregnatedwith the resin blend and dried, producing a B-stage prepreg having aresin content of about 27.6 percent, a volatile content of about 0.8percent and zero flow when measured at 250° F under 15 psi. The threeplies are oriented with the warp at about 0°, 30° and 60°, respectively,against a polyvinyl alcohol/wax release treated plaster tool surface.The lay-up is covered with a layer of perforated Teflon fluorocarbonrelease film and three plies of 181 glass fabric bleeder, and curedunder full vacuum (at least about 25 inches of mercury) for about 1 hourat about 250° F. The cured acoustic laminate is tested for airflowresistance using an airflow velocity of about 16.95 cm/sec., giving avalue of about 26.2 rayls (cgs). The acoustic laminate has excellentacoustic properties when bonded to a honeycomb core with an imperviousbacking.

EXAMPLE III

A catalyzed resin solution is prepared as described in Example II. Threesheets of Type 7533 glass fabric are impregnated with the resin anddried, producing a B-stage prepreg having a resin content of about 26.3percent. The three plies are stacked on an aluminum tooling surfacecoated with Frekote 33 release agent.

The warp of each ply is oriented to produce a 0°, 30°, 60° arrangement.The stack is covered with a sheet of perforated Teflon fluorocarbon filmand three plies of 181 glass fabric bleeder. The resin is fully curedunder full vacuum, greater than 25 inches of mercury, at about 250° F.for about one hour. An airflow resistance of about 26.5 rayls (cgs) isobtained when tested at an airflow velocity of about 16.95 cm/sec.

EXAMPLE IV

Twelve sheets of type 7533 glass fabric are impregnated with an epoxyresin and cured to the B-stage as described in Example II. Resin contentof the dried prepreg is about 27.8 percent. A first set of four of thesheets is oriented with the warp at 0°, 30°, 60° and 90° against apolyvinyl alcohol/wax released tool surface. Another set is stacked on asimilar surface with warp orientation of 0°, 22°, 44° and 66°. A thirdset is stacked on a Frekote 33 released aluminum surface with warporientations of 0°, 22°, 44° and 66°. Each stack is fully cured asdescribed in Example II. The first set gives an airflow resistance ofabout 25.0 rayls, the second about 29.8 rayls and the third about 26.6rayls, all at an airflow velocity of about 16.95 cm/sec.

EXAMPLE V

A sheet of a three dimensional woven 3-layer glass fabric, having an 11by 11 count construction available from Woven Structures, Inc., isimpregnated with a blend consisting of equal amounts of two Bisphenol-Aepoxy resins, available from the Shell Chemical Co., under the Epon 828and Epon 1001 designations and a stoichiometric equivalent amount of aneutectic composition of two aromatic diamines, p,p'-methylene dianilineand m-phenylene diamine, available from the Shell Chemical Co., underthe Curing Agent Z designation, in a methyl ethyl ketone solvent. Theprepreg is flash dried by heating for about 10 minutes at about 200° F.Resin pickup, dry, is about 21.8 percent. The prepreg is fully curedagainst a polyvinyl alcohol/wax release treated tool surface under about1 psi pressure for about 2 hours at about 200° F. The cured acousticlaminate is found to have an airflow resistance of about 16.9 rayls atan airflow velocity of about 16.95 cm/sec. The laminate is then bondedto a one inch thick, 2.2 pcf reinforced phenolic hexagonal honeycombcore, by coating the core edge with a paste epoxy adhesive from the 3MCo., then pressing the laminate thereagainst for about 90 minutes atabout 240° F. and about 12 psi. An impervious sheet of epoxy laminate isbonded to the back surface of the core. The resulting panel is found tohave excellent acoustic properties and high structural strength.

EXAMPLE VI

A sheet of Type ES-1189 glass fabric (21 percent open area), availablefrom Woven Structures, is impregnated with a blend consisting of twoBisphenol-A epoxy resins, available from Shell Chemical Company underthe Epon 828 and Epon 1001 trade marks in a ratio of 75 to 25 parts byweight, and a stoichiometric equivalent amount of eutectic compositionof two aromatic diamines, p,p'-methylene dianiline and m-phenylenediamine, available from Shell Chemical Company under the Curing Agent Zdesignation, in a methyl ethyl ketone solvent. The prepreg is flashdried by heating for about seven minutes at about 200° F. Resin contentof the prepreg is about 17 percent. The prepreg is fully cured betweentwo sheets of fluorocarbon release film (available from duPont under theTeflon trademark) placed between metal caul plates by heating for about2 hours at about 200° F. under about 30 psi pressure. The cured acousticlaminate is found to have an airflow resistance of about 2.8 rayls at anairflow velocity of about 16.95 cm/sec. The laminate is then bonded to0.663 inch thick 3/8-inch-5056 aluminum 0.003 inch Dura-Core, availablefrom Hexcel, with an unsupported film adhesive from American CyanamidCompany which is perforated and reticulated in position on the core. Thelaminate is pressed thereagainst and an aluminum sheet is simultaneouslybonded to the back surface of the core with a support film adhesive byheating at about 350° F. under about 40 psi pressure for about 60minutes. The resulting sandwich panel is found to have excellentacoustic properties and high structural strength.

EXAMPLE VII

A sheet of ES-1189 glass fabric, available from Woven Structures, isimpregnated with a blend consisting of equal amounts of solidBisphenol-A epoxy resin and a tetrafunctional tetraphenylmethane typeepoxy, available from Shell Chemical and Ciba-Geigy under the Epon 1001and ERRA 0163 trademarks, respectively, and a stoichiometric equivalentratio of HET-anhydride (hexachloro endomethylene tetrahydrophthalicanhydride), available from Hooker Electrochemical Company, in a methylethyl ketone solvent. The prepreg is dried by heating for about 10minutes at about 285° F. Resin content of the prepreg is about 20percent. The prepreg is cured for about 1 hour at about 285° F. underabout 50 psi pressure between fluorocarbon coated glass fabric bleederplies available from Taconic Coated Fabrics under the Armalondesignation plus one ply of 181 glass fabric bleeder. The laminate ispostcured for about 2 hours at about 400° F. A high temperatureresistant laminate results with good acoustic properties and a highstrength-to-weight ratio.

EXAMPLE VIII

Type 1532 glass fabric (ECG 150, 2/3 plain weave, 16 × 14 count)available from J. P. Stevens Co., is impregnated with a blend consistingof a liquid cycloaliphatic epoxy resin and a solid Bisphenol-A epoxyresin, available from Ciba-Geigy under the Araldite CY 178 and Araldite6060 trademarks, respectively, in a ratio of 75 to 25 parts by weight,and a 0.9 stoichiometric equivalent ratio of hexahydrophthalicanhydride, available from National Aniline Division, Allied ChemicalCorporation, plus 12 parts per hundred resin of an organometalliccatalyst (sodium alcoholate), available from Ciba-Geigy under thedesignation of Accelerater 065, in a methyl ethyl ketone solvent. It isthen heated at about 200° F. until a volatile content less than 1.0percent and less than 1% flow at about 250° F. under about 5 psi isobtained. Resin content is about 26 percent. Four sheets are laid upwith the warp of each successive sheet oriented at 0°, 15°, 30°, 45° toone another against a tool surface coated with Frekote 33 release agentfrom Frekote, Inc. The layup is covered with a layer of perforatedTeflon fluorocarbon release film and three plies of 181 glass fabricbleeder, and cured under about 10 inches of mercury vacuum for about 2hours at about 250° F. plus about 2 hours at about 300° F. The laminatehas good acoustic characteristics when bonded to a honeycomb core withan impervious backing.

Although specific conditions and ingredients have been described in theabove examples of preferred embodiments, these may be varied and otheradditives, such as the flow control agents discussed above may be used,where suitable. Other modifications and variations of this inventionwill occur to those skilled in the art upon reading this disclosure.These are intended to be included in this invention, as defined by theappended claims.

I claim:
 1. An acoustical laminate comprising at least two sheets ofopen weave glass fiber fabric impregnated with from about 13 to 32weight percent cured epoxy resin, based on glass fabric weight, saidlaminate being porous to the extent of having an airflow resistance ofabout 2 to 60 rayls at an airflow velocity of about 17 cm/sec and havinga thickness of from about 0.02 to 0.10 inch.
 2. The acoustic laminateaccording to claim 1 wherein said glass fabric comprises at least twosheets of a leno weave glass fabric.
 3. The acoustic laminate accordingto claim 1 wherein said epoxy resin comprises a Bisphenol-A type resin.4. The acoustic laminate according to claim 1 wherein said epoxy resincomprises a Bisphenol-A Novolac type resin blend.